On the invoke-diff-cmd branch: Fix error messages.  Add comments to
document non-canonical path usage.

* subversion/libsvn_client/diff.c 

  (diff_content_changed): Add comment explaining the presence of
    non-canonical path in function call.

* subversion/svn/io.c

  (svn_io_run_external_diff): Fix layering violation in first error
    message.  Remove CR from second error message.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1485004 13f79535-47bb-0310-9956-ffa450edef68